The present invention relates generally to metal rolling mills having automatic gage control capabilities and more particularly to an improved method and apparatus for determining the initial roll opening of such a mill when automatic gage control is to be utilized.
Metal rolling mills employ a pair of opposed rolls through which a piece of metal, hereinafter referred to as a workpiece, is passed to reduce the metal in thickness. Prior to the passing of the workpiece, however, the mill must undergo what is commonly called a "setup." That is, the basic operating parameters of the mill must be determined. In most modern mills, the setup is under the control of a computer which is supplied with statistical data of many types in order to determine the operating parameters. As an example of the computer controlled mill, reference is made to U.S. Pat. No. Re. 26,996, "Computer Control Systems for Metals Rolling Mill" by R. G. Beadle et al, issued Dec. 8, 1970 and assigned to the assignee of the present invention.
One of the most basic determinations that must be made in any mill setup, particularly in the case of a single stand reversing mill such as will be more fully described hereinafter, is that of the initial unloaded roll opening; that is, the spacing between the rolls in the unloaded condition which occurs prior to the entry of the workpiece between the rolls. It is well known that the unloaded roll opening is less than the thickness or gage of the workpiece that will emerge from between the rolls in the rolling process. This is because the mill stand is not a perfectly rigid apparatus and it does exhibit what is known as "stretch." Because the mill stand is not perfectly rigid, when the metal workpiece is between the rolls the forces occasioned by the rolling spread the rolls apart by some amount. There are many known methods and techniques for determining the initial roll opening. All, however, employ the same basic considerations which normally include: (1) the desired output thickness, (2) the mill stretch, (3) the material composition and temperature, (4) the workpiece width and (5) the effective roll crown.
The normal prime objective of any rolling operation is to produce "on gage" material; i.e., a sheet of metal which is of the desired thickness throughout its length. In a single stand reversing finishing mill, the workpiece is passed repeatedly in opposite directions through a single pair of work rolls with the unloaded roll opening being successively reduced so as to repeatedly take reductions in the thickness of the workpiece. In such mills, the concept of automatic gage control (AGC) has long been known and employed. It has for many years been recognized that the force separating the rolls can be related to thickness reduction and further that a measurement of this force can be used to adjust the rolls while the metal is actually being rolled to thereby improve the control of the output gage. As examples of this fundamental knowledge, reference is made to U.S. Pat. No. 2,726,541, "Measuring Apparatus For Rolling Or Drawing Sheet Or Strip Metal" by R. B. Sims, issued Dec. 13, 1955 and to U.S. Pat. No. 2,680,978, "Production Of Sheet and Strip" by W. C. F. Hessenberg et al, issued June 15, 1954.
Two basic types of AGC are known today. These are commonly referred to as the "lock-on" system and the "absolute" system. In the lock-on system, the first observed gage is used as a reference and the adjustments to the roll opening are made using this first observed reading as the reference point. For reasons which will be more apparent as this description proceeds and particularly with respect to the description of FIG. 3, because the ends of the workpiece are normally of lesser thickness than the central portion, it is quite easy for the lock-on system to reference an incorrect gage and thus roll the complete workpiece "off gage." It is possible, in the lock-on system, to wait until a fair amount of material has passed through the rolls before taking the reference reading. This, obviously, results in an off gage portion near the end of the workpiece which results in waste. The absolute system employs a preestablished value which is used as a reference point. Because of entry shock, however, it is necessary to delay at least 0.1 seconds before initiating the gage control, and, because of limitations in the speed of response of the roll gap positioning system, another 0.2 seconds or more will elapse before any significant gage correction results. Thus, once again waste is potentially inherent in the system. As an example of the absolute system, reference is made to U.S. Pat. No. 3,906,764, "Rolling Mill Control Method And Apparatus" by G. E. Mueller, issued Sept. 23, 1975.
Regardless of the type of system, lock-on or absolute, and the present invention may be used with either basic system, the initial setup referred to above is provided. In the prior art, this initial setup, roll opening in this particular instance, is based upon the evaluation of force readings made on the next preceding pass taken throughout the main body portion of the workpiece and ignores the end portions because of the relatively extreme deviations therein. Thus, there is a strong possibility of a relatively large percentage of waste when it is considered that the total finished length of a workpiece after rolling in a single stand reversing finishing mill may be typically 100 to 150 feet and often less than 60 feet. Thus, if the entry rolling speed is 11 feet per second, as is typical and as was indicated for the absolute system, three-tenths of a second is lost, then it is seen that some 3 to 4 feet may be lost at the end of the piece which is off gage and may wind up as waste.
One additional general point should be made with respect to systems employing AGC. In a mill employing this system, the total reduction from the initial to the final thickness may take several passes through the mill. It is not unusual for a single workpiece to be passed through the stand eleven or twelve times in this process. The automatic gage control feature, when available, is not normally used at all times but is used in the later passes where, customarily, the amount of reduction is relatively less. That is, in the earlier passes of the workpiece through the roll, a heavier reduction will be taken and AGC is not used although setup is employed in each case. As the workpiece nears the end of its rolling schedule, the amount of reduction taken during a single pass is reduced and the AGC system may be employed in the last three or four passes.